Optical lenses, and in particular ophthalmic lenses, require very high quality standard, therefore very high quality manufacturing process is to be used in order to obtain high quality optical lenses.
Historically, optical lenses have been manufactured by different processes such as cast molding.
Cast molding requires the use of two complementary molds in which the lens material is added by gravity casting. These molds present a specific design corresponding to the desired lens design.
Lenses made by cast molding suffer a large number of quality defects during solidification due for example to shrinkage. Shrinkage may cause surface voids and the non-adherence of the final product to the lens design.
In order to assure the quality of the lenses, it is required to have a method for checking the quality of the product lenses.
Different methods for checking the quality of the product lenses by cast molding can be used.
The one by one method consists in checking the product lenses one by one. Such a method does not meet demands of low-cost, high-volume, high-yield rapid production.
A more effective method consists in checking the quality of the molds to be used. An attempt consists in correlating the quality of the manufactured lenses to the quality of the mold. Advantageously, such a method is more compatible with the requirements of low-cost, high-volume, high-yield rapid production. But such a method can give information corresponding to only one manufacturing step.
Furthermore, the molding method presents limitation in terms of quality, cost and is limited in the number of different designs it offers.
Therefore, new manufacturing techniques such as digital surfacing are used.
In the art of lens manufacture, a finished lens is usually made from a semi-finished lens blank or from a finished uncut lens.
Semi-finished lens blanks have usually optically finished front surfaces; however, the back surfaces of these blanks need to be generated and fined. Then, they are either polished and/or coated to produce finished uncut lenses. Finished uncut lenses are then edged to the proper frontal shape and edge contour to fit into spectacle/glasses frames or other mounting structures.
Single vision lenses that are outside the normal range of inventoried finished uncut lenses and most multifocals, namely progressive addition lenses, are made from semi-finished lens blanks. Semi-finished lens blanks are made with various front surface curve radii, and have various topographies including spherical, aspheric, hyperbolic, irregular aspheric such as progressive addition lenses, and polyspheric such as executive type segmented bifocals and trifocals.
Finished uncut lenses are lenses that are optically finished on both front and back surfaces and only need to be edged to the proper shape and edge contour to become finished lenses. Most optical laboratories keep an inventory of single vision finished uncut lenses in various powers, sizes and materials to take care of most of the more common single vision ophthalmic prescriptions. Progressive addition lenses are nowadays possibly manufactured as finished uncut lenses by using a 3D machining usually called “digital surfacing”.
To generate a desired prescription for a lens, calculations are required to determine the topography of the surfaces of the lens, namely its back surface if semi-finished lens blank is used. Such calculations typically involve variables that include the front surface radii of the semi-finished blank, the index of refraction of the lens blank material, prescription values of the desired lens, statutory values regarding minimum lens thickness, and the physical dimensions of the frame or mounting structure.
In the art, various means can be used to accomplish the physical process of producing a back surface of optical quality. Most of these methods begin by generating a back surface that approximates the desired back surface topography and surface smoothness using digital surfacing. This approximate surface is then fined to a more perfect approximation in both curvature and surface smoothness.
After the appropriate accuracy and smoothness is achieved in the fining process, the surface is then polished and/or coated to produce a surface of optical quality. The optically finished lens blank is then edged to the proper shape and edge profile to fit into or with the frame for which the lens was made.
The quality of the lens produced using a digital surfacing process requires to be checked. The lens that is produced using the digital surfacing process corresponds to a given prescription. Therefore a quality checking method that would check the quality of a small fraction of the produced lens would not be reliable.
The solution commonly used is to check the quality of each individual lens as they are produced. However as for the molding manufacturing process such method does not meet demands of low-cost, high-volume, high-yield rapid production.
Therefore, there is a need for an efficient method to assure the quality of the lenses products, in particular for lens produced using manufacturing process that does not involve molds. Thus, the goal of the present invention is to provide a process for controlling the quality of the manufactured lenses.